1. Field of the Invention
The present invention relates to a volume hologram layer-transferring foil for use in transferring a volume hologram layer onto a transfer-receiving member, a method of producing a volume hologram laminate using the same, and others.
2. Description of the Related Art
Hologram is an object of the wave front of an object light recorded on a photosensitive material as interference fringe by interference of two lights (object and reference beams) of the same wavelength, and, if a light at the same wavelength and of the same angle as the reference beam used during interference fringe recording is irradiated, a diffraction phenomenon caused by the interference fringe is induced, regenerating the wave front identical with that of the original object light. The hologram, which has many advantages such as favorable appearance and relative difficulty in copying, has been used, for example, in security applications. In particular, it is used widely on plastic cards such as credit card and cash card, mainly for prevention of copying and for favorable appearance in design.
Such holograms may be divided into several groups, typically into surface relief hologram and volume type hologram, depending on the method of recording interference fringe. The surface relief hologram is a hologram which is recorded by a fine convexo-concave pattern engraved on the surface of a hologram layer. On the other hand, the volume type hologram is a hologram wherein the interference fringe generated by light interference is recorded as fringes different in refractive index three-dimensionally in the thickness direction of the hologram layer. In particular, the volume type hologram, in which a hologram image is recorded, based on the difference in refractive index of the material, has an advantage that it is more difficult to copy than the relief hologram, and thus, is expected to find application as an antiforgery means, for example, for use on valuable stock certificates and credit cards.
Examples of the known methods of engraving a hologram on valuable stock certificate, card or the like, when the hologram is used for purposes such as the means of improving appearance and preventing forgery, include a method of weaving slit-shaped holograms therein and a method of embedding a hologram in a medium in the state visually recognizable from outside, but generally used is a method of bonding the hologram onto a particular position. In particular, a method of transferring a hologram from a hologram transfer foil carrying a hologram previously formed on any substrate and thus bonding the hologram onto a particular position is used widely as a simple and easy method.
The hologram transfer foil is a material in the configuration comprising a substrate, a hologram-carrying hologram layer and a thermally adhesive heat seal layer formed thereon in that order, and the hologram layer is transferred onto a transfer-receiving member via the heat seal layer. Such hologram transfer foils are disclosed, for example, in Japanese Patent Application Laid-Open Nos. 2003-316239 and 2005-3809.
Such a hologram, which is difficult to copy, has been used as an antiforgery means in various applications; but methods facilitating copying of the hologram are becoming available recently, and simple use of a hologram is pointed out to be insufficient as an antiforgery means. For that reason, there is an increasing need for a hologram transfer foil that gives a hologram improved in antiforgery function and thus insecurity.